This invention relates to an input power level control technology of an optical transmission device used for an optical transmission system, especially to an input power level control technology of an optical transmission device that directly receives an input from optical fiber, such as of an optical amplifier and an optical transmitter receiver.
Optical transmission devices, such as an optical amplifier, have characteristics of wideband and low noise. The optical amplifier is important, especially in the optical transmission system of the WDM (Wavelength Division Multiplexing) method, in order to collectively amplify optical signals of a plurality of wavelengths and thereby increase a transmission length, and is widely used in optical transmission systems.
FIG. 16 shows a configuration example of an optical transmission system using optical amplifiers. In this example, to simplify its explanation, only one-way transmission from a node A 701 in the left of the figure to a node D 704 in the right is shown.
Output lights of wavelengths λ1-λn from an optical transmitter 100 are wavelength-multiplexed by a multiplexer 501 and sent out onto an optical fiber transmission line 600. The multiplexed lights that reached the receiving side are demultiplexed by an optical demultiplexer 502 for demultiplexing wavelength-multiplexed lights, which are received by optical receivers 200 capable of receiving the respective wavelengths λ1-λn.
In the optical fiber transmission line 600, there are provided a repeater optical amplifier 350 that is a node B 702 for compensating a loss of the optical fiber transmission line 600, an optical add/drop multiplexer 503 that is a node C 703 for dropping and adding only some wavelengths (λi), and the like. Moreover, for each of the node C 703, and the node D 704, receiving optical amplifiers 400 each of compensating a loss in the optical fiber transmission line 600 are used. For the node C 703, a transmittion optical amplifier 300 compensating a loss of the optical add/drop multiplexer 503 is used.
Because of differences among the lengths between nodes, loss characteristics, the numbers of optical connector connections, etc., the optical fiber transmission line 600 may exhibit a wide range of the transmission loss. For this reason, optical transmission devices, such as optical amplifiers, that receive the input directly from the optical transmission line 600 are required to have a wide dynamic range of input.
For example, in the case where an Er (erbium) doped optical fiber amplifier is used for an optical amplifier, the gain and output optical power of the optical amplifier are determined by the Er concentration and a length of the Er-doped optical fiber and pumping optical power used. Then, gain saturation depending on the input power to the optical amplifier limits the dynamic range of input.
Generally, since the WDM optical amplifier needs to perform an operation of constant gain regardless of change in the number of wavelengths to be amplified, the input dynamic range is often set to about 5 to 7 dB. Therefore, if an input power range of about 20 dB is required in order to cope with various transmission line losses, it is necessary to adjust the optical input power level using a separate optical attenuator.
FIG. 17 is a functional block diagram of an optical amplification board 11 that constitutes an optical amplifier. The optical amplification board 11 has an optical amplification unit 40, an optical amplification unit control circuit 41 for controlling the optical amplification unit 40, and an optical input-connector 20 and an optical output connector 30 that interface with the optical fiber 11.
Since generally the optical amplification board 11 used for an optical transmission device does not have a function of controlling an optical input power of the optical amplification unit 40, it is necessary to install a fixed or variable optical attenuator 801 externally and adjust the optical input power.
It is often the case that a loss of an optical transmission line is unknown until starting up the transmission device. Therefore, when installing a fixed optical attenuator externally, it is necessary to prepare many kinds of fixed optical attenuators beforehand, which will force complicated and uneconomical stock control of fixed optical attenuators.
When installing a variable optical attenuator externally, it is necessary to manually adjust a variable optical attenuator, while measuring the optical input power of the optical amplification unit 40. For this reason, there are problems that installation cost of the device may arise and reliability may fall because of artificial mistakes.
It is natural that securing a space to install a fixed optical attenuator or variable optical attenuator is required. For the installation place, it is necessary to choose a place that gives good workability for installation/removal and adjustment of an optical attenuator.
The above-mentioned subject is the same also in an optical transmission device that receives an input directly from the optical fiber: transmission line 600, such as a transponder reception unit and a wavelength multiplexer input unit, as well as the optical amplifier.
In order to accommodate various optical interfaces flexibly, a detachable (pluggable) optical transceiver module has been proposed and is being used widely in optical transmission systems (for example, see JP 2004-363948 A). As a device to solve complexity in installing and removing a fixed optical attenuator, there is a plug-type attenuator.
Moreover, as a device to resolve a labor in adjusting a variable attenuator, there is an optical amplification board equipped with an automatic adjusting mechanism for the attenuation (see JP H11-17259 A). FIG. 18 shows an optical amplification board 12 equipped with an automatic adjustment mechanism. The optical amplification board 12 shown in FIG. 18 is the optical amplifier board 11 shown in FIG. 17 added with a structure for controlling an input optical power to be kept at a constant. In the optical amplifier board 12, light inputted from the optical input connector 20 through the optical fiber 11 is inputted to the optical amplification unit 40 through a variable optical attenuator 802 and the optical splitter 52. Light partly branched by the optical splitter 52 is detected by the opto-electronic conversion circuit 53, and fed back to the variable optical attenuator 802 by a constant-input-power control circuit 54 that controls the detected optical power so that it is kept at a specific value.
Since an interface of the plug-type attenuator is an optical connector, it cannot be made variable. On the other hand, the optical amplifier board 12 can perform automatic control of an optical input power to the optical amplification unit 40. However, the variable optical attenuator 802 and the optical amplification unit 40 were integrated into one piece to form the optical amplifier board 12. Therefore, even when the variable optical attenuator 802 is unnecessary, it cannot be removed. Especially, the variable optical attenuator 802 has an insertion loss of about 1-2 dB even when the attenuation is set to a minimum. Accordingly, when the variable optical attenuator 802 is unnecessary, this corresponds equivalently to a case where the noise factor of the optical amplification unit 40 becomes worse by 1-2 dB. Although the loss is as small as only 1-2 dB, this directly affects OSNR (Optical signal-to-Noise Ratio) of an optical amplification repeater system, and reduces a number of repeater stages or a transmission distance by 20-40%, resulting in increased number of stages.
For OSNR, please refer to the following expression described also in ITU-T Recommendation G.692.OSNR=Pout−L−NF−10 Log(N)−10 Log(hνΔνo)                Pout: Output power (dBm)        L: Span loss (dB)        NF: Noise factor (dB)        N: Number of spans        h: Planck's constant        ν: Optical frequency        Δνo: Optical bandwidth        
The present invention was made in view of the above-mentioned situation, and aims at providing an optical transmission device with a simplified configuration that can attenuate optical input power only when it is necessary so that it falls within a range and suppress the transmission loss when it is unnecessary.